1. Field of the Invention
The invention described herein is directed to the manipulation of multiple objects suspended in fluids within microfluidic devices through the application of complex force fields. More specifically, the present invention is a method for determining and subsequently applying a set of signals to one or more actuators on a microfluidic device so as to respectively apply a corresponding force on each of one or more objects contained therein to thereby manipulate the position, velocity, shape, orientation, and/or distribution thereof.
2. Description of the Prior Art
Microfabrication techniques have been used for over a decade to produce a variety of submillimeter mechanical structures. For example, the new fabrication techniques have led the way to the production of Micro-Electro-Mechanical Systems (MEMS) in which microscopic machinery, sensors, actuators, and electronic circuitry are assembled on, and in many cases etched from, a common substrate, such as silicon. Many of these micromachined devices have enjoyed a wide range of applicability in such fields as chemical and biological research.
In certain technological fields, such as the aforementioned chemical and biological research, the physical scale of certain domains of interest have motivated the development of sophisticated equipment capable of manipulating microscopic objects, both individually and in selected groups. One device widely used in this area is the optical tweezer (also known as laser tweezers) which uses laser light to manipulate objects of molecular size scales. Optical tweezers create an optical trap on an object through light scattering forces and light intensity gradient forces of a focused laser beam. The forces combine to hold the object in the center of the focused laser spot. The trapped object may be repositioned by moving the focused laser spot as desired.
Optical tweezers are effective in manipulating certain types of objects, but suffer several shortcomings which prevent their implementation in a wider range of applications. Optical tweezers can be used to manipulate particles, provided there is a difference in the index of refraction of the particle and that of the surrounding medium, but have yet to be used effectively to redistribute fluids. Manipulation of fluids on a microscopic scale has become a useful method in delivering chemical agents to individual cells to observe how the cells react thereto.
Another shortcoming of optical tweezers is that they are large and generally expensive pieces of equipment. In typical applications, a laser tweezer will consist of one or more lasers, a microscope, and high-quality focusing optics to produce each optical trap.
Manipulation of submillimeter objects by electrophoresis and other methods using an applied electric field have been used for many years. Electrophoresis has been widely used for separating and sorting particles into bands in accordance with particle size and inherent electric charge. Gel electrophoresis, for example, whereby an electric field is applied to molecules suspended in a porous gel, is used in the field of genetics for DNA profiling. However; while electrophoresis is useful for applying a force on certain particles, such as molecules, the process is not operative on objects immovable by an electric field, such as objects made of a dielectric material. Additionally, electrophoresis may be used to sort particles according to a charge/size ratio into sorting bins located along a straight path, but does not provide a means for steering the objects toward alternative locations not on the path.
Sorting of minute particles is a prevalent requirement in many research and biochemical fields, and many means for performing this task are widely available. For example, one common application passes a stream of particles suspended in an electrolyte through a small aperture over which an electric field is applied. A particle in the aperture displaces an amount of electrolyte equal to its own volume. In accordance with the Coulter principle, the volume displaced changes the impedance of the aperture and is measured as a voltage pulse, the height of which is proportional to the volume of electrolyte displaced, i.e., the volume of the particle. The particles may then be sorted by size by deflecting different sized particles into a corresponding sorting location or bin, by some mechanism such as an optical tweezer. The Coulter particle sorter illustrates the benefits of device implementation on a micromachined platform, i.e., to measure a change in impedance in a channel or aperture caused by the presence of a microscopic particle, the channel or aperture is required to be formed on a size scale comparable with the size of the particle.
An illustrative example of another cell sorting device constructed by micromachined techniques is provided by the journal article, “An Integrated Microfabricated Cell Sorter,” by Anne Fu, et al. (Analytical Chemistry, Vol. 74, No. 11, Jun. 1, 2002). The referenced cell sorter implements a network of microvalves and micropumps for controlling the movement of cells suspended in a fluid after the classification thereof by controlling the surrounding fluid flow. Cells within the device are classified by means of fluorescence of the cell resulting from excitation by an argon laser. Various valves and pumps are activated in accordance with one of a number of predetermined patterns so as to direct a particle to a destination sorting bin by directing the flow of the suspending fluid along one of a number of predetermined paths. However, the geometry of the fluid channel and the pump and valve configuration allow only a limited control over the motion of any particular cell. Moreover, the configuration does not afford simultaneous parallel control of multiple particles within the fluid. For example, the device does not contemplate directing different objects toward each other.
Particle placement and sorting are not the only tasks for which microscopic object manipulation means are desired. Many applications require the manipulation of fluids on a microscopic scale for purposes of, for example, mixing, dosing, and delivering small quantities of drug to individual cells. In other applications, objects such as strands require shape orientation or conformation. For example, in certain applications, DNA strands may need to be “unwrapped” to expose certain structural features for study. To perform these functions, complex motion of multiple particles, strand segments and surfaces is necessary. However, simultaneous arbitrary control of the trajectories of multiple objects presents a challenging controller design problem.
One device for controlling the motion of multiple objects is the Universal Planar Manipulator (UPM) developed by Dan Reznik, formerly of the University of California at Berkeley. Objects to be manipulated are placed on a rigid, horizontally oriented plate. The plate is coupled to one or more actuators which vibrates the plate in the horizontal plane. The objects are moved by means of frictional forces selectively overcome or engaged by the acceleration of the vibrating plate.
The control of motion of the objects on the UPM is achieved through a closed loop configuration consisting of a camera, for photographing the horizontal plate and the objects thereon, a set of motors for vibrating the plate and a computer for a) determining the positions of the objects at each sampling interval, b) computing the forces to be applied to each object so that the object follows its predesignated trajectory, c) computing the motion of the plate which will bring about all of the required forces, and d) applying a signal to each actuator so as to move the plate in the required manner. The process is repeated periodically according to a predetermined sampling schedule.
The UPM control method determines, at each sample period, a center of rotation (COR) about which the plate is to be rotated and the magnitude (i.e., duration) of the rotation. By strategically placing the COR at each sampling period, the required forces are generated, in a time-averaged sense, so as to move the objects in their respectively assigned trajectories.
Whereas the UPM illustrates that parallel control of multiple objects on a common medium is possible, its method of control cannot be applied to systems where gravity has much less influence on the objects than do other forces. For example, in fluidic realms, the effects of turbulence and fluid viscosity are as significant as those due to gravity. Fluid flow, in general, is a complex process presenting exceptional control challenges. Some of the unwanted effects of turbulence may be mitigated by controlling the fluid on a small size scale where the momentum of the fluid reaches negligibility. However, the control of fluid flow by acceleration (i.e., by relying on gravitational forces) on such size scales becomes highly impractical.
As shown by consideration of the shortcomings of the prior art, there is an apparent need for parallel control of multiple objects suspended or immersed in a fluid such that each object follows an arbitrary trajectory.